1. Field of the Invention
The invention relates to a method and an apparatus for electromagnetically controlling the rotation of a movable element around its axis.
2. Description of the Prior Art
The concept of moving an element housing a permanent magnet is well-known and it has been used in a variety of applications.
One of these applications is the rotation of a movable element around its axis, as has been disclosed in European application Ser. No. 87104786.6 in the name of Alternative Energy Research Center (the present applicant), which is specifically referred to for a preferred embodiment.
In the cited disclosure a cylinder housing a permanent magnet is revolved around its axis by the selective activation of a plurality of "U" shaped electromagnets located on a supporting plate positioned near the cylinder base housing said permanent magnet. The locations of the electromagnet poles (which are substantially perpendicular to the visualizing surface of the panel) correspond to the positions where the rotating cylinder should be able to stop; when one of the electromagnets is activated by its related electrodes, the cylinder is driven by the permanent magnet to the selected position.
The inherent disadvantage of this apparatus mainly resides in the existence of a plurality of electromagnets which may be magnetized by the permanent magnet to such an extent that the rotating element could stop in a wrong position.
To overcome this problem, a pulsed feeding could be used, but this would involve additional problems because of the necessity of a pulse generator and the related pulse amplitude and duration calculations.
Another disadvantage of the disclosed apparatuses is that misalignements of the permanent magnet can occur; that is, the permanent magnet can stop in a position which is offset by several degrees from the required position. These misalignements can dramatically and negatively affect the operation of the whole device, particularly if said device is used for displaying information.
Moreover, a drive control such as that required by the cited application has some other specific requirements:
the coils, being expensive and not easily assemblable elements, should be as few as possible;
the coils should be machine-windable on the related poles also when the electromagnets are inserted in a matrix panel;
the electrodes should be easily connectable with the other circuitry elements;
each position should be reachable from any other position with a single movement;
each selected position should have a stable equilibrium and there should be no other stable equilibrium positions;
the dispersed flux should be significantly reduced such as to avoid any influence on the adjacent magnet equilibriums;
the needed peak power for the rotation should not exceed 1.5 Kwatt/m.sup.2 ;
the electromagnet stator should have a simple geometric configuration, in order to simplify the winding of the coils, and should be easily produced, i.e. by injection molding or deep-drawing;
the stator materials should have high permeability and magnetic induction at saturation.